1) first soak in water for 2 days, even if there is paint can not care, anyway, to be thoroughly wet.

2) Then flatten it with flat floor tiles or marble, you can add weights on it appropriately, make sure to safeguard enough to flatten the wooden disk.

3) Keep the pressure and let the gas dry naturally.

4) Now in winter, it may take 10 or so days, after drying is flat, and does not bounce.

5) Finally you can spray a clear varnish to prevent moisture later.